1. Field of the Invention
The present invention relates to eddy-current testing, a non-destructive testing method for inspection of metallic structures, and, more particularly, to means for reducing noise from eddy-current defect indications to enable analysis of the electric signal to accurately and consistently evaluate the metallic structure, such as heat exchanger tubing material, being tested.
2. Background Information
Eddy-current testing is a widely used, non-destructive testing method for inspection of tubing material. The basics of eddy-current testing are set forth in Fundamentals of Eddy Current Testing, by D. J. Hagemaier, published by The American Society for Nondestructive Testing Inc, 1990, which is incorporated herein by reference.
The speed, sensitivity, and ease of use of eddy-current testing make it an ideal choice for inspection of heat exchanger tubing, which may be non-ferromagnetic and thin-walled. However, even in light of all the benefits associated with eddy-current testing, it is often considered one of the most frustrating non-destructive testing methods. Although the technique employed in eddy-current testing is simple and reliable, inspection data and results are frequently contradictory and misleading due to the many material variables and measurement noise which influence the eddy-current measurements. Common problems associated with eddy-current testing include inconsistent results obtained with successive inspections of the same material and inconsistent analysis of results by different analysts. As a result, confidence in the technique suffers.
Most of the difficulties associated with eddy current testing arise from the analysis of the data generated and the quality of the data. Eddy-current testing by its nature is sensitive to any change in the electrical or magnetic properties of the test part. In particular, when inspecting heat exchanger tubing, noise effects and random fluctuations caused by support structures, electrically conductive deposits, permeability variations, dents and bulges, roll expansions, and other phenomena, as well as actual defects, will be exhibited in the resulting test data. Thus, indications from these noise effects and random fluctuations frequently combine with the indications from the defects such that both the detection and sizing of flaws are compromised. In fact, present data acquisition and analysis techniques require that a human analyst recognize and quantify the defect indications imbedded in the noise which is unrelated to the defects being assessed.
An analyst, or operator, interprets eddy-current test data by viewing and inspecting visual displays of impedance plots or Lissajous patterns. The eddy-current testing measures complex impedance of the metallic object being tested. The complex impedance has a real and imaginary component which is measured as an output voltage by the instrumentation. These voltage quantities are represented by the Lissajous pattern, which is a waveform having an X-Y plot as a function of time or displacement. A typical Lissajous pattern is normally produced by recording orthogonal components of a two-dimensional process. For example, a Lissajous pattern can be produced by sampling the X and Y components of an electric field, either as a function of time or displacement. Thus, Lissajous patterns are generally two-dimensional waveforms in which real and imaginary components of successive points in a collection of data points are plotted in an X-Y plane. Such a plot forms lobes radiating from an origin with the angular position of the lobes relative to the origin representing the phase angle.
In the field of non-destructive testing, Lissajous patterns are created by passing eddy-current probes along metallic structures to detect anomalies. The Lissajous patterns revealing anomalous features are then interpreted by the analyst, or operator, who observes and analyzes significant geometric characteristics, such as phase and amplitude, and qualitative parameters, such as the fatness of a figure, to evaluate the nature of the physical structure. For example, an operator would visually inspect the Lissajous patterns in the impedance plane and identify specific pattern classes, such as a "figure-8" class or a "figure-V" class. Then, once the class is identified, the operator would determine such features as the pattern phase and amplitude, which for outside diameter defects are the quantitative measure of defect depth.
Operators have developed a highly refined ability to observe the shape and phase angle of most Lissajous patterns, such as eddy-current test patterns, and determine therefrom, with a reasonable degree of confidence, the characteristics of the two-dimensional process under observation, i.e., whether defects exist in the material being tested and the types of defects which are present. However, it must be appreciated that, for the most part, Lissajous patters are not perfect geometric patterns, and, as such, sometimes great difficulty is experienced in interpreting them either visually or with conventional pattern recognition algorithms.
Eddy-current test patterns are frequently replete with "noise". Noise is defined as curvature changes that occur along a curve's arclength that are unrelated to overall pattern appearance. For instance, a curve may oscillate rapidly, tending to confound the pattern analysis.
Three different types of noise have been identified. The first type is random noise, which is a consequence of fluctuations that occur during the measuring process. The reduction of the effect of this noise is the object of this invention. The second type of noise, called endpoint noise, is the consequence of the imprecise determination of indication limits in the data; points not associated with the indication may be included at the ends of the curves. The third type of noise may occur anywhere along a pattern curve where a small loop appears. These loops are believed to be unrelated to physical characteristics, but nonetheless display large curvature change at all levels of resolution. Pattern analysis by the operator begins after all three types of noise have been removed from the data.
In order to effectively utilize conventional visual or numerical methods to interpret features of the eddy-current test pattern which contains a relatively large component of random noise, it is apparent that the random noise should first be eliminated or substantially reduced to a relatively low level so as to smooth out the irrugular curvature of the waveform.
There are several different known prior art methods for reducing random noise fluctuations in two-dimensional waveforms. Arguably, the best of these methods uses a spline curve which is fit to a small group of points lying on the waveform. In the spline curve method, one central data point of this group is then moved to lie on the spline curve. The process is progressively moved along the waveform until all of the points have been moved. However, the spline curve method is unsatisfactory in that it removes information from the two-dimensional waveform, thus distorting it; this method results in a reduction in overall waveform quality.
Other methods for reducing random noise fluctuations in two-dimensional waveforms include one of two processes, undersampling or filtering, which remove unwanted voltage components of the eddy-current response voltage. Undersampling is a method whereby fewer samples of the eddy-current signal are made than are necessary to capture the most detailed portion of the signal. This method does not enhance the signal in any way and results in a signal generally deficient in small scale features.
Filtering is a method which imposes an a priori model of the data on the eddy-current signal. However, there is no physical reason for the imposition of any model on this type of data. Filtering methods remove information; in contrast, the present invention makes use of an oversampled signal by redistributing information to the low curvature part of the signal at the expense of the high curvature component.
Through the use of multiple-frequency eddy-current testing systems, modern equipment is capable, in principle, of acquiring the necessary data to correctly diagnose all indications. Applying consistent analysis techniques, however, is required to achieve proper evaluation of the test data. Methods in use prior to this invention do not allow consistent analysis of test data nor make use of all the information present in the data. Current data acquisition and analysis techniques require that the interpreter recognize and quantify defects imbedded in noise unrelated to the defects being assessed.
To overcome the above-mentioned obstacles, a method has been developed that effectively removes the effect of unwanted noise components of the electronic signal, allowing the defect-related portion of the signal to remain. This has the effect of making the defect pattern clear so that unambiguous analysis and diagnosis can be made. In addition, the resulting clear shape of the defect pattern enables reliable quantitative measurements of the pattern to be made.
The creation of noise-free data patterns through the present invention has enabled automatic data evaluation since computer pattern recognition methods are more likely to succeed with well formed patterns rather than with noisy ones. Machine pattern recognition, the emulation of the visual skills of an analyst, in turn, decreases the reliance on a human operator, which enhances the evaluation of eddy-current data.
Accordingly, it is a general object of the invention to provide a method for removing the effect of unwanted noise from eddy-current test signals produced during inspection of tubing material.
Another object of the present invention is to provide consistent inspection data from eddy-current tests and, thus, provide consistent and reliable evaluation of the tubing material being tested and identification of any defects present therein.
Another object of the present invention is to provide eddy-current test patterns from which quantitative measurements of the pattern can be made.
It is another object of this invention to provide an eddy-current testing system which is capable of removing the effect of unwanted noise from eddy-current test signals produced during inspection of tubing material for accurate and complete analysis of the eddy-current test measurement signals.
Other objects, advantages and novel features of the invention will be apparent to those of ordinary skill in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.